A large number of internal voltages of different magnitude are required in integrated semiconductor circuits, for example, in dynamic semiconductor memory modules, so-called DRAMs, in order to supply the internal functional units and to operate them correctly. The output voltage must be as constant as possible and must be provided with adequate current driver capability, with as low an impedance as possible.
As is known, a DRAM comprises memory cells with a storage capacitor, whose state of charge represents the stored information. Due to leakage currents, the stored charge state in the capacitor is changed, and the separation from a reference decreases. In order to make it possible to read the stored information without any errors despite this, it is necessary for the reference levels to be used to be as constant as possible and to maintain a predetermined level of magnitude, even in poor operating states. For example, a voltage generator is required which is located precisely centrally between the voltage levels that represent the two binary logic states. Since the information to be read is compared with this central voltage level, its accuracy is subject to relatively stringent requirements. Finally, further potentials, which supply the memory cell array and the circuits for reading and writing are also provided by a higher-level voltage generator arrangement.
A voltage generator arrangement such as this comprises two or more stages. A bandgap reference circuit provides an output potential, which is referred to as reference ground potential and is largely independent of external operating influences, such as the external supply voltage or temperature. The bandgap reference circuit has a high-impedance output. The bandgap reference circuit is thus expediently followed on the output side by an impedance converter, which transforms the reference potential, that is provided with a high impedance, to a low impedance. Finally, the impedance converter drives a voltage generator, which is arranged on the output side and supplies an output potential that is relatively constant and has a high current driver capability, and whose magnitude is set as a function of the output signal from the impedance converter. Two or more impedance converters may be driven in parallel by the same bandgap reference circuit, or various output-side voltage generators may be provided in order to produce different output voltages, or the same voltages, which can be fed in at different points on the semiconductor chip.
In the case of a voltage generator arrangement such as this, it has been found to be expedient to provide separate reference ground potential lines. In this case, the bandgap reference circuit and the impedance converter are connected to a first reference ground potential line. The bandgap reference circuit and the impedance converter draw a constant current irrespective of the various operating states of the DRAM. Furthermore, the current that is drawn is relatively small. The voltage drop along this line is thus constant, or can easily be compensated for. The output-side voltage generator is connected to a second reference ground potential line, which is separate from the first. The two reference ground potential lines are, for example, formed from metal tracks which run in a metallization plane on the semiconductor chip and which, for example, are composed of aluminum or of an aluminum alloy. The reference ground potential is supplied from the exterior via what is referred to as a connecting pad. Various pads are also feasible, which are then connected to one another externally to the chip. The two reference ground potential lines are typically connected via the connecting pad at least to the external supply for the reference ground potential.
Since the current, which is not inconsiderable during operation, is supplied via the external voltage generator to a load that is to be driven, and this current flows back via the second reference ground potential line to the connecting pad, in which case the current that is drawn can also fluctuate relatively severely as a function of the operating states of the DRAM, the voltage drop along the second reference ground potential line is no longer negligible. A voltage drop is thus produced between the connecting pad and that point at which the output-side voltage generator makes contact with the second reference ground potential line. This voltage drop can fluctuate over time.
The described voltage generator arrangement is thus subject to the problem that the reference generator and the impedance converter are always supplied with a constant reference ground potential, while the potential at the reference ground potential connection for the output-side voltage generator fluctuates as a function of the current flowing via the second reference ground potential line. Thus, during operation, the reference ground potentials for the output-side voltage generator on the one hand and for the bandgap reference circuit and the impedance converter on the other hand differ from one another. Until now, the output-side voltage generator has raised the reference voltage that is supplied from the impedance converter to a higher voltage level. For example, the bandgap reference circuit produces an output voltage of 1.2 V, and the impedance converter produces an output voltage of 1.6 V. The latter output voltage is raised by the output-side voltage generator to, for example, 2.0 V. The output-side voltage generator thus amplifies the voltage drop that occurs on the second reference ground potential line and, in consequence, amplifies the voltage error within the output voltage that is to be produced.
In particular, as miniaturization of the structures on the integrated semiconductor chip progresses and as complexity of the circuits to be supplied increases, there is a trend on the one hand to reduce the internal voltages further although, on the other hand, higher currents are required, even though the resistances of the metallization lines increase as a result of the smaller structure widths. The reference ground potential lines are becoming relatively longer with respect to the number of functional units to be supplied, as integration progresses. As a consequence of these boundary conditions, it is problematic to provide the required internal voltages with sufficient constancy and a sufficiently high current drive capability with the use of conventional concepts. The amplification of the parasitic voltage drop along the second reference ground potential line in the output-side voltage generator also results in the output voltage becoming less stable.